(a) Technical Field
The present disclosure relates to a fuel control system and method for a fuel cell system. More particularly, it relates to a fuel control system and method for a vehicle fuel cell system, which can efficiently supply hydrogen to a fuel cell stack and increase the efficiency of an ejector.
(b) Background Art
A fuel cell system, which may be applied to a hydrogen fuel cell vehicle as an environmentally-friendly vehicle, generally comprises a fuel cell stack for generating electricity by an electrochemical reaction between reactant gases, a hydrogen supply system for supplying hydrogen as a fuel to the fuel cell stack, an air supply system for supplying oxygen containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, and a thermal management system for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function.
The hydrogen supply system comprises a hydrogen supply unit (e.g., a hydrogen tank) from which hydrogen as a fuel is supplied to the fuel cell stack, a hydrogen supply valve (e.g., a start/stop solenoid valve) for supplying and cutting off the hydrogen from the hydrogen supply unit, a pressure regulator for regulating the pressure of hydrogen supplied, and a hydrogen recirculation system for recirculating unreacted hydrogen discharged from an anode outlet of the fuel cell stack to an anode inlet.
The hydrogen recirculation system may be configured with only an ejector or with both an ejector and a hydrogen recirculation blower. When the hydrogen is recirculated and reused (with the hydrogen recirculation blower), the distribution of reactants in the fuel cell stack becomes uniform due to an increase in the flow rate of hydrogen in the fuel cell stack, and thus a uniform cell voltage distribution can be obtained, which allows the fuel cell stack to operate more stably.
The method of using the hydrogen recirculation blower, however, has various problems. For example, it has a complicated structure, requires lubrication, and generates noise and vibration. Therefore, various techniques for recirculating hydrogen using the ejector without a hydrogen recirculation blower have been proposed.
FIG. 4 is a diagram showing an example of the configuration of a hydrogen supply system in a vehicle fuel cell system, in which an ejector is used for hydrogen recirculation.
In the above hydrogen supply system, a typical hydrogen tank 11 as a hydrogen supply unit stores high-pressure hydrogen and supplies the high-pressure hydrogen to a fuel cell stack 20, and the high-pressure hydrogen supplied from the hydrogen tank 11 passes through a pressure regulator 12, a hydrogen supply valve 13 (e.g., a start valve), and an injector, a pressure control valve (e.g., a solenoid valve), or a pressure control actuator 14.
At this time, the pressure of the high-pressure hydrogen supplied from the hydrogen tank 11 is reduced to pressure P1 by the pressure regulator 12, regulated again to pressure P2 by the injector/pressure control valve/pressure control actuator 14, and is then supplied to the fuel cell stack 20 together with the recirculation gas sucked by an ejector 16.
In FIG. 4, the pressure of hydrogen (e.g., a mixture of fresh hydrogen and recirculation gas) supplied through the ejector 16 is shown by P3.
Moreover, a purge valve 22 for removing impurities from an anode of the fuel cell stack 20 is provided in an anode exhaust line 21 of the fuel cell stack 20.
Pressure P1 regulated by the pressure regulator 12 is always maintained constant. At pressure P1, hydrogen in an amount that can be used over the entire output range of the fuel cell vehicle should be supplied to the fuel cell stack 20, and thus a relatively high-pressure is required.
The ejector 16 serves to supply hydrogen at low pressure P3 to the fuel cell stack 20 and to suck and recirculate the unreacted hydrogen from the anode of the fuel cell stack 20 using low pressure created by high-speed hydrogen jet while the high-pressure hydrogen passes through a nozzle (e.g., a converging nozzle or a converging-diverging nozzle).
Next, the features and drawbacks of the injector, the pressure control valve, and the pressure control actuator 14, which are disposed in front of the ejector from the hydrogen tank, will be described.
1. Injector: An injector should cover the entire output range of the fuel cell system, and thus pressure P2 should be regulated below pressure P1. While the injector has a very fast response and a long life span compared to the pressure control valve, it is difficult to develop the injector due to a large change in pressure at front and rear ends thereof. Moreover, since the amount of hydrogen supplied is controlled by high pressure P1, the frequency of operation of the injector is increased, which reduces the life span of the injector. Prior art patents related to the injector include U.S. Pat. No. 7,320,840, U.S. Patent Publication No. 2009/0155641, etc.
2. Pressure control valve: A solenoid valve using proportional control is typically used. The amount of hydrogen supplied is determined by controlling pressure P2 based on the output of the vehicle. The hydrogen supply is performed by controlling pressure P3 according to a request of a controller. Since the injector controls the hydrogen supply in an open/close manner, the hydrogen supply to the ejector is discontinuous, and thus it is inefficient for the hydrogen recirculation through the ejector. However, the pressure control valve can continuously control the hydrogen supply, and thus it can be efficiently used in the hydrogen recirculation through the ejector. Moreover, the pressure control valve should quickly control the amount of hydrogen supplied according to the rapidly changing vehicle output, but the response time is slow due to the nature of the pressure control valve. Furthermore, the pressure control valve should continuously operate according to the operating conditions of the vehicle, and thus the durability of the pressure control valve is reduced. In addition, to follow pressure P3 required by the controller, the pressure control valve should be controlled under proportional-integral-derivative (PID) control, which makes the pressure control valve operate more frequently, thereby further reducing the durability. During low output operation, a small amount of hydrogen should be supplied, and thus it is necessary to precisely control the flow rate (or pressure P2) of hydrogen. However, a large amount of hydrogen flows due to high pressure P1, even when the pressure control valve is minimally opened, and thus it is not easy to control the pressure control valve. Especially, to allow the pressure control valve to maintain airtightness, the pressure control valve should be pressed with relatively strong force when it is closed. In this case, it is more difficult to precisely control the flow rate of hydrogen. Moreover, it is difficult to control the pressure control valve due to its hysteresis characteristics.
3. Actuator/motor: This is efficient for the hydrogen recirculation through the ejector. Moreover, since there is no hysteresis and location information can be accurately detected, it is easy to control the actuator. However, the actuator is disadvantageous in terms of cost and weight and in terms of durability due to the fact that a complex gear structure is included in the actuator.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.